The importance of electrochemical processes such as the evolution of chlorine and the evolution of oxygen cannot be overemphasized. Chlorine evolution, one of the world's largest industrial electrochemical processes, involves the electro-oxidation of chloride ions to produce chlorine, sodium chlorate, sodium hypochlorite or hypochlorous acid, depending upon the cell design and operating conditions. Oxygen is the product of the electro-oxidation of water molecules and oxygen evolution is coupled with most of the commercially significant industrial processes occurring in aqueous electrolytes, e.g., electroplating, electrowinning, metal recovery and water electrolysis.
Since the 1970s so-called mixed metal oxide electrodes have transformed the technological and economical aspects of processes involving both oxygen evolution and chlorine evolution. A mixed metal oxide electrode includes two kinds of metal oxides, such as an oxide of a valve metal, (e.g., titanium or tantalum) and an oxide of a platinum group metal (e.g., ruthenium, iridium or platinum). Many combinations of platinum group metal oxides and valve metal oxides have been prepared and characterized, but presently it is primarily mixtures of TiO2—RuO2, TiO2—RuO2—IrO2, TiO2—RuO2—SnO2, TiO2—IrO2 and Ta2O3—IrO2 that are used for the various commercial electrochemical processes. The commercial success realized by mixed metal oxide electrodes is largely due to their properties, i.e., good electro-catalytic properties, a high surface area, good electrical conductivity, as well as excellent chemical and mechanical stability during extended operation in aggressive environments.
Electro-catalysis is broadly defined as the ability of an electrode to influence the rate of the electrochemical reaction. This involves a physical and/or chemical interaction between the electrode surface and the electro-active species that diffuse and migrate to that electrode surface. It is this interaction, which almost exclusively involves the oxide of the platinum group metal in mixed metal oxide electrodes, that reduces the energy required to drive the reaction, effectively lowering the electrode potential and therefore the overall cell voltage. Thus, the power consumed by the electrochemical process is reduced. The high surface area of the mixed metal oxide electrode effectively lowers the applied current density and hence the electrode potential and cell voltage, again resulting in a reduction in the power consumed for the process. Similarly, the electrical conductivity of the electrode structure can be important, minimizing the resistance to current flow through the structure, i.e., reducing the ohmic overpotential which is a component of the cell voltage.
The distribution of the platinum group metal oxide in the coating affects both the electrochemical activity and the conductivity of the electrode. The valve metal oxide is essentially non-conductive, so that the electrical conductivity depends on the particles of the platinum group metal oxide, as is discussed in a review entitled “Physical Electrochemistry of Ceramic Oxides” by S. Trasatti [Electrochimica Acta, 36(2), 225-241 (1991)]. The morphology of the layer has been shown to affect its conductivity, e.g., compact layers are more conductive than “mud-cracked” layers, the latter being typical of the morphology of commercially available mixed metal oxide electrodes. Conductivity is also affected by the thermal program used in the manufacture of the electrode.
The platinum group metal oxide particles within the coating provide the electro-catalytic activity, particularly towards the oxidation of inorganic ions, such as the chloride ion, water molecules (oxygen evolution) and towards the oxidation of aliphatic and aromatic organic molecules. The porosity of commercially available mixed metal oxide electrodes and topcoated electrodes is believed to be important, allowing electro-active species easy access to the catalytic sites. U.S. Pat. No. 6,251,254 (issued Jun. 26, 2001) describes the formation of a porous layer on the surface of a coating containing iridium oxide to provide an anode for use in the electroplating of chromium from chromium OM ions. U.S. Pat. No. 7,247,229 (issued Jul. 24, 2007) describes the addition of a porous topcoat that allows water molecules access to the catalytically active layer underneath, but inhibits the diffusion of large organic molecules or large inorganic ions to those sites. This electrode is described as being useful as the anode in electroplating, electrowinning and metal recovery processes. The application of a porous topcoat over a mixed metal oxide coating is also the subject of U.S. Pat. No. 7,378,005 (issued May 27, 2008), which describes an electrode for the production of dilute aqueous solutions of ozone for disinfection and sterilization processes. In this patent, the porosity of the topcoat is developed specifically in the thermal process used in forming the topcoat, heating the coated substrate to temperatures ranging from 600° C. to 700° C. Furthermore, it is argued that the porosity obtained in this way is critically important to the generation of ozone in the electrolysis of the aqueous solutions. U.S. Pat. No. 7,156,962 (issued Jan. 2, 2007) discloses an electrode for production of ozone or active oxygen in for-treatment water by electrolysis. The electrode has an electrode catalyst surface layer formed on the surface of a conductive substrate, wherein the electrode catalyst surface layer contains a noble metal or metal oxide.
However, the porous nature of the topcoat and the generation of gas within the pores of the topcoats described in U.S. Pat. Nos. 7,247,229 and 7,378,005 can lead to mechanical instability during extended operation. The topcoat may become powdery and may be displaced from the electrode surface. Furthermore, the roughness of the surfaces of the intermediate layer and the topcoat may increase the active surface area and consequently lower the current density and therefore the potential at which the electrode operates. In the production of strong oxidants, such as hydrogen peroxide and ozone, it is believed to be more efficient to operate at the higher anodic potentials.
Recently there has been interest in the development of anodes which are less catalytic towards the oxygen evolution reaction, allowing operation at high anodic potentials in aqueous electrolytes for the production of strong oxidants, such as hydrogen peroxide and ozone. Furthermore, advanced oxidation technologies are being developed for the destruction of organic contaminants in industrial wastewater. Direct electro-oxidation using high overpotential electrodes offers a possible approach and antimony-doped tin oxide and boron-doped diamond are considered as candidate materials for this application. It is claimed that hydroxy radicals are formed at the surface of the boron-doped diamond electrode and these radicals rapidly oxidize a wide variety of organic contaminants in water. There is also evidence, presented by Comninellis et al, [J. Electrochemical Society, 150(3), D79-D83, (2003)], that recombination of the hydroxy radicals at the electrode surface results in the formation of hydrogen peroxide. Presently however, neither tin oxide nor boron-doped diamond electrodes are used commercially. It has been shown that the stability of tin oxide is limited and the large-scale manufacture of diamond coated titanium substrates has proven to be difficult and costly.
It would be advantageous to avoid the use of a topcoat by manufacturing an electrode having a coating comprised of multiple mixed metal oxide layers, wherein the concentrations of the platinum group metal and valve metal vary as the thickness of the coating increases. Furthermore, it would be advantageous to form a relatively smooth coating that is less porous than the typical mixed metal oxide coating. Such an electrode could be tailored to a specific application, be it the production of strong oxidants such as ozone or hydrogen peroxide or as an oxygen evolving anode in electroplating processes, wherein the oxidation of additives such as levelers and brighteners is effectively inhibited, or as an oxygen evolving anode in water treatment and wastewater purification processes. Moreover, it would be advantageous to manufacture such an electrode by established, large-scale, cost-effective methods. The present invention is directed to a multi-layer mixed metal oxide electrode and method for making the same that provide these and other advantages.